1. Field of the Invention
This invention relates generally to a timer circuit, and is more specifically directed to an improved timer circuit for controlling an electric device, such as a battery-operated radio or the like, so that the electric device may be maintained in its ON state for a predetermined time and then automatically turned OFF.
2. Description of the Prior Art
Electric devices, such as battery-operated portable radios, are often provided with a timer circuit to turn the radio ON upon actuation of a switch, and then to automatically turn OFF the radio after a predetermined length of time has elapsed. Bedside radios are conveniently provided with such timer circuits in order to provide music for a predetermined period of time and then to automatically turn OFF after the listener has fallen asleep.
In conventional timer circuits for use with battery-operated devices, as aforesaid, a storage capacitor is charged to the full battery potential, and when a timer switch is operated, the storage capacitor is discharged through a discharge resistor which, with the storage capacitor, forms an RC time constant circuit. The time constant of the RC time constant circuit determines the delay time, that is, the time required for the voltage stored in the storage capacitor to decay from its initial value to a triggering level at which the electric circuit from the battery through the radio is broken. However, after the battery has been used for an extended period, the voltage that it can supply to the storage capacitor drops significantly. The resulting lower voltage stored in the storage capacitor reduces the delay time between switch actuation and the time when the radio is turned off, so that the delay time becomes shorter and shorter as the battery voltage decreases with continued use.
A previous improvement in timer circuits for the above described purpose uses a constant voltage circuit to limit the charging voltage for the storage capacitor to a voltage which is less than the battery voltage. This constant voltage circuit normally includes a number of forward-biased diodes which are connected in parallel with the charging capacitor during the time that the actuating switch is closed. While this improved timer circuit exhibits a substantially constant delay time between switch actuation and shut off during the life of the battery, it has the disadvantage that, for any given RC time constant circuit, the delay time will be significantly lower than with a conventional timer circuit.
Moreover, it is not possible to extend the delay time of the above improved timer circuit merely by increasing the values of the resistive and capacitive elements of the RC time constant circuit. If the value of the storage capacitor, or the value of the discharge resistor is increased to compensate for the inherently shorter delay time of the improved timer circuit, certain problems can arise. For instance, if a storage capacitor of exceptionally high value is used, the leakage current across the capacitor can interfere with the operation of the timer circuit. In addition, high-value storage capacitors tend to be overly bulkly and prohibitively expensive, and thus can be impracticable for use with a small portable radio or the like. Alternatively, if an exceptionally high value resistor is selected as the discharge resistor, the discharge current through the discharge resistor will become insignificant in relation to stray leakage currents across the capacitor, such as the leakage current in a thyristor or similar device which is commonly used to turn OFF the electric device when the voltage on the capacitor reaches the triggering level. Thus, an increase in the resistance value of the discharge resistor will not significantly contribute to an increase in the time constant of the RC time constant circuit.